1. Field of the Invention
This invention relates to a spatial light modulator system. The invention has particular, although not exclusive relevance to this a spatial light modulator system for use in a projection system in which a projected display is formed by modulating light from a light source by a spatial light modulator device, and then projecting the modulated light on to a display area in which beams having different color content are modulated by different spatial light modulator devices each driven in accordance with a different video signal, and the modulated beams are combined to form a single projected color display.
2. Description of the Related Art
A spatial light modulator is an optical component which is controllable to modulate an incident light beam. A relatively old example of such a spatial light modulator is the eidophor, a layer of oil scanned by a cathode ray. A more recent example of spatial light modulators is an active matrix device, comprising a matrix of individually addressed pixel light valves or modulators; the liquid crystal modulator array described in, for example, EP 0401912 is one modulator array of this type. In EP 0401912, a liquid crystal matrix is provided in a light path to transmit in a variable manner, and hence amplitude modulate, an incident light beam (without altering its path or optical axis). Another example of an active matrix device is the tiltable mirror array, the so called "deformable mirror device" described in, for example, U.S. Pat. Nos. 4,856,863, 4,615,595, and 4,596,992.
Such deformable mirror devices comprise an array of miniature mirrored cantilever beam elements each carrying electrodes so as to be electro statically deflectable between two positions. The extent of the deflection can be controlled by the applied electrostatic potential to provide variable degrees of reflection, or the device can be operated in a binary manner by applying predetermined electrostatic potentials to switch between discrete deflection states. Each element thus angularly deflects the incident light beam and hence changes the optical axis of the light beam.
By using an array of such elements, each individually addressable, a two dimensional image can be reproduced by exposing the array to an incident light beam, modulating the incident beam by controlling the individual mirror devices from a picture signal, and collating the beam reflected in a particular direction. The small size and fast switching times of such deformable mirror devices makes them usable at video picture data rates, enabling the display of television or video moving images on a display screen onto which the collated beam is projected.
The incident beam is not scanned, as is an electron beam, but illuminates the entire device. In order to display a color image, therefore, it is necessary to provide three separately illuminated arrays of mirrored cantilever beam elements, each array being responsive to a different primary color or primary color combination, and to combine optically the modulated beams reflected from each array onto a single optical display.
One example of an application of such a system is in large scale displays as disclosed in our earlier International Applications WO91/15843, WO91/15923, PCT/GB92/00002, and PCT/GB92/00132, and PCT/GB93/00456 published as WO92/12506, and WO92/13424, and WO93/18620 respectively (all of which are incorporated herein by reference).
In PCT/GB93/00456 published as WO93/18620, the described embodiment has a pair of colour dependent splitter and combiner surfaces, so that an incoming white light beam is split into a first pair of beam components, one of the pair of beam components then being further split into a second pair of beam components. For example, the first dichroic surface may reflect a blue beam to a first tiltable mirror array, and the transmitted beam is then split by the second dichroic surface into a reflected red beam and a transmitted green beam. Each of the red, blue and green beams is then supplied to a respective tiltable mirror array, which modulates the respective beam, and the modulated beams are then recombined in the same manner by the dichroic surfaces (although they could be combined by different dichroic surfaces in principle).
The coloured beams impinging on the tiltable mirror array are symmetrical and hence the number of reflections that they have been through is irrelevant. However, the beams modulated by the arrays have a defined "handedness", and each time the beam is reflected the "handedness" will be reversed. To combine all three modulated beams into a single colour image, the handedness of each must be the same. However, where the same surfaces are used both to split and to combine the beams as described above, then it is necessary to provide that the modulator modulating the green beam does so to generate a mirror reversed image, since the green beam is arranged to be transmitted through both dichroic surfaces without reflection, whereas the red and blue beams are each reflected once.